The invention relates to a sensor for the measurement of physiological parameters. The invention relates further to a method for the measurement of physiological parameters.
The measurement of one or more physiological parameters of the human body is increasingly gaining in significance. Thus, a combination sensor for the combined measurement of the oxygen saturation of the haemoglobin in arterial blood and also of the arterial partial pressure of carbon dioxide is known from the document EP 0 267 978 A1. For the measurement of the oxygen saturation (SpO2) a non-invasive, optical, and generally known method is used which is termed pulse oximetry. A pulse oximeter system of this kind comprises a sensor which is applied to a location of the human body with a good blood supply, a pulse oximeter, and also a connection cable which connects the sensor to the pulse oximeter. For the measurement of the CO2 concentration in the blood, the transcutaneous carbon dioxide partial pressure (tcpCO2) is determined with the aid of an electrochemical measuring apparatus. Detailed information concerning these generally known measurement methods are, for example, to be found in the following review article: xe2x80x9cNoninvasive Assessment of Blood Gases, State of the Artxe2x80x9d by J. S. Clark et al., Am. Rev. Resp. Dis., Vol. 145, 1992, pages 220-232. Details of the pulse oximetry measurement method are for example to be found in the document WO 00/42911.
Disadvantages of the sensor disclosed in the document EP 0 267 978 A1 are the facts
that disturbing signals which arise falsify the measurement,
that the sensor has to be calibrated frequently,
that the sensor has a relatively thick cable,
that the sensor together with the evaluation device is relatively expensive,
and that the sensor only permits relatively simple measurements to be carried out.
According to some embodiments of the present invention a sensor is provided that may include at least one measuring apparatus and also a digital sensor signal processor which is arranged in the sensor and which is connected in a signal conducting manner to the measuring apparatus and which makes available a digital output signal. According to some embodiments of the present invention, a method may be provided for the measurement of physiological parameters in which an analog measured value of a measuring apparatus is detected by a sensor, the analog measured value is converted into digital values in the sensor and the digital values are supplied to a signal evaluation device.
The object is in particular satisfied by a sensor for the measurement of physiological parameters such as oxygen or carbon dioxide in the blood comprising at least one measuring apparatus and also a digital sensor signal processor arranged in the sensor which is connected in signal-conducting manner to the measuring apparatus and which makes the measured values available in digital form for the further processing.
The sensor of the invention has a digital sensor signal processor which digitises the signal measured by the measuring apparatus, so that this signal is available in the sensor for further processing in digital form. A digital sensor signal processor of this kind is also referred to in English as DSSP (Digital Sensor Signal Processor) or as Single Chip MCU (MiroComputerUnit). A sensor signal processor of this kind comprises not only a microcontroller with memory, microprocessor and interfaces on a single chip, but rather also an analog-to-digital converter and also a digital-to-analog converter. The digital sensor signal processor enables, amongst other things, the values measured by the measuring apparatuses arranged in the sensor to be converted into digital values within the sensor. From this, the following advantages arise in particular:
The measured, analog signal is converted within the sensor into a digital signal which is insensitive to disturbance. This also permits weak analog signals to be measured cleanly.
The signal is digitally transmitted between the sensor and a subsequent evaluation device, which extensively precludes signal falsification due to stray radiation.
The evaluation device can be arranged within the sensor or spaced apart from the sensor.
Two conductive wires are sufficient for the transmission of the digital data. The connection cable between the sensor and the evaluation device can thus be made very thin.
A digital signal processing is essentially carried out in the evaluation device. This enables a favourably priced evaluation device obtainable as a standard product, with a processor and matched software corresponding to the sensor.
In an advantageous design of the sensor, the data transmission takes place in cableless manner between the sensor and the evaluation device, for example by means of electromagnetic waves.
In a further advantageous embodiment the sensor is designed such that its digital output signals have fixed normalised values. In a preferred embodiment a reference curve, for example a calibration curve of the measuring apparatus arranged in the sensor is stored in the sensor. Such sensors have the advantage that no time-consuming or costly calibration is required on changing the sensor, since all sensors have a specified output signal.
In a further advantageous embodiment the sensor has a circuit board which is equipped with the essential or all required electronic components. A sensor of this kind can be manufactured at extremely favourite cost.
In a further advantageous embodiment the sensor has an inner space which is electrically screened against the outside, which yields the advantage that disturbing signals cannot or can hardly be superimposed on the measured signals.
It is of central importance that the sensor of the invention also allows weak signals to be measured unambiguously and that the measured signals can be supplied free of disturbance to a signal evaluation device. This has the consequence that no complicated method is required for the signal evaluation in order to unambiguously and reproducibly evaluate an otherwise normally noisy signal.
It has proved to be particularly advantageous to use the sensor of the invention on the ear, in particular on the earlobe. Moreover, it is particularly advantageous to heat the sensor with a heating device in order to thereby keep the earlobe at a reproducibly constant temperature. The earlobe proves to be a particularly advantageous measurement position, because the ear is located relatively close to the heart with respect to the blood circulation, substantially closer than the finger cap, which is, for example, also suitable for the measurement of oxygen or carbon dioxide in the blood. Moreover, as a result of the heating, hardly any vascular construction or hardly any vessel narrowing arises at the ear by reason of the heating. The sensor of the invention thus enables measurement to be made at the earlobe with a very low Signal to Noise (S/N) ratio, so that a measurement signal of excellent quality is available.
This high signal quality now in turn enables further physiological parameters to be determined from the measured values, such as, for example, the blood pressure, which is measured by means of the so-called CNIBP method, which signifies in English xe2x80x9cContinuous NonInvasive Blood Pressurexe2x80x9d. In this connection the systolic blood pressure is, for example, determined by means of the pulse oximetric measurement method which can be measured at the earlobe, as is for example described in detail in the following document: xe2x80x9cCan Pulse Oximetry Be Used to Measure Systolic Blood Pressure?, R. Chawla et al., Anesth Analg, 1992; 74:196-200xe2x80x9d. The sensor of the invention permits the pulse curve shape, for example the so-called plethysmogram to be measured in that the oxygen content of the blood is measured pulse oximetrically for example 50 or 100 times per second and the blood pressure is determined from the resulting shape of the curve.
The high signal quality enables the hemotacrit, abbreviated internationally with xe2x80x9cHCTxe2x80x9d to be determined as a further physiological parameter. The determination of this parameter is, for example, disclosed in detail in the document U.S. Pat. No. 5,803,908.
The sensor in accordance with the invention is also suitable for the measurement of the composition of the respiratory air.
In a further advantageous embodiment a data memory is arranged in the sensor in which measured values or patient data can for example be stored. In an advantageous embodiment this data memory is made sufficiently large that data measured over a longer period of time can be stored in the sensor, for example in a nonvolatile memory, also termed an EEPROM.
In an advantageous method, the measured values are supplied to a signal evaluation device arranged after the sensor and the evaluated data are supplied at least partly to the sensor again and stored in its nonvolatile memory. The sensor can also be connected to different signal evaluation devices temporally one after the other, which respectively store the evaluated data in the memory of the sensor. A signal evaluation device can, if required, also access the data stored in the memory, for example the patient data.
The sensor in accordance with the invention is, for example, also suitable for the longer term monitoring of patients, for example also in emergency situations, in which the sensor is, for example, secured to the patient""s ear and the sensor remains continually with the patient and the data of the signal evaluation devices, to which the sensor is respectively connected one after the other, is at least partly stored. Thus, it is possible with a single sensor to continuously monitor a patient substantially without gaps, starting for example at the site of an accident, and subsequently in the ambulance, in the operating theatre through to the wake-up station. Thanks to the fact that all the measurement data is available at every station and, moreover, that even further patient data is eventually available, reliable data is also available at any time in emergency situations, which enables an ideal care of a patient.